The goal of this research is to establish a scientific basis for developing targeted interventions that can improve gait function in the elderly. Walking speed is a common clinical measure of gait function. Slow preferred walking speed has been associated with falls, can contribute to physical disability and is one of the primary criteria used to describe the clinical syndrome of frailty. Reduced strength and diminished aerobic capacity are commonly proposed causes for slow walking. Frail elderly adults can often make significant improvements in muscle strength and endurance through exercise. However, such gains have not consistently translated to improved gait performance. These findings are indicative of the complexities of gait performance and indicate the need for a more fundamental understanding of the neuromuscular factors that limit gait speed. [unreadable] This study will employ musculoskeletal models and forward dynamic simulations to describe the causal relationship between muscle actions and movement during gait. These novel analyses will be used to identify limiting factors, describe compensatory mechanisms and predict the affects of specific interventions. Aim 1 will rigorously examine the contributions of muscle coordination, strength, posture and passive joint stiffness to reduced gait speed in healthy old adults. We will test the hypothesis that age-associated reductions in gait speed arise primarily due to increased passive stiffness at the hip and only secondarily due to diminished muscle power at the ankle. Aim 2 will identify the subject-specific factors that limit gait speed in elderly who exhibit intermediate frailty. We hypothesize that identifiable subject-specific limitations in either passive hip flexion stiffness, ankle plantarflexor power capacity or hip abduction weakness will be the primary factors limiting gait speed in elderly who walk slowly. [unreadable] The development of forward dynamic simulations that elucidate how muscles contribute to both normal and impaired movement, as proposed here, is an important step to understanding the causes of slow walking among frail elderly. The results of this study are fundamental to the development of effective interventions that can improve gait function and thereby contribute to the maintenance of mobility in elderly adults [unreadable] [unreadable]